Thought I'd share some thoughts on the 161 as I've been sailing it quite a lot over the last few months.
One thing I've noticed with this board, is that it is less sensitive to fin stiffness than previous model starboards. I find that I can get "almost" similar results with a S- or an M fin in most conditions under 15knots. At the bottom end of the wind range (<10knots), the softer fins are better, but the difference isn't as pronounced as you would think. I've mainly been testing with an R19 S- and an R13 73cutdown M, and can get almost identical results upwind, with the R19 being a little faster downwind with its smaller tip.
This makes me understand why an R17 won't work in this board, and what fins you should really be using...
Basically, the extra width in the tail of the 161 is contributing to your lift upwind. Because the board is so big (and wide), you need a high-lift fin to unstick the board and get it to rail. High-lift fins can come in two forms, either a soft fin (soft fin relies on twist to get the lift), or a fin with a large area (like an R19). This is why I think you can get a M or a S-/S-- fin to do a similar job, as long as the M you are using is high in area (R19, R13 cutdowns). *** Although in saying that, a high area fin like an R19 that is super soft, should give you the greatest lift as it combines the best of both worlds, but a softer fin is usually a little slower than the same fin in M stiffness downwind.
The R16 and R17 fins are not high-lift fins. They have thinner cross sections and less area than R19/R13s and rely on speed to get the lift. Despite whatever you do to get the nose to lift on the 161 (ie, boom up, mast-track back, footstraps back), these fins simply don't generate enough lift to get this big board to rail and unstick the nose: more board in the water means more drag, which is slow.
So Remi is right on the money with having your mast track further back, straps in the back holes and not using an R17 in this board. Your main aim on this board to go fast is to get the nose up and the board railing...